1. Field of the Invention
The present invention relates to a method or polishing both sides of an optical lens, in which the lens is mounted in a first workpiece holder and polished on a first side. The invention also relates to a device for polishing both sides of optical lenses.
2. Description of the Related Art
Optical lenses are produced out of transparent materials by several grinding processes or other material-removing machining methods. Mineral glass or suitable plastics can be used. In every case, at least one polishing process, by means of which the surface roughness of the lens is reduced to such an extent that the amount of light scattered at the surface becomes negligible, is carried out after the above-cited machining processes which give the lens its basic shape.
In the methods conventionally used today, the time required to polish the lenses is about twice as long as the time required for the preceding machining processes to shape the lens. It is therefore an especially important development goal to reduce the polishing time. Because the polishing time itself, i.e., the time during which the polishing tool is in actual contact with the lens, cannot be reduced in any immediate way because of the nature of the process, the idea of reducing the nonproductive times becomes especially significant.
According to the state of the art, lenses are polished by means of the following technology:
Polishing machines have a pivoting head at the top, which carries the tool spindle, to the lower end of which the polishing tool, usually a shaping tool, is attached. The pivoting head itself is able to pivot around the so-called "B axis" and is attached to an X slide, which can execute horizontal movements perpendicular to the B axis. In the lower part of the polishing machine there is a Z slide, which can move vertically and which carries the workpiece spindle, the axis of which is vertical. The axes of the tool spindle and the workpiece spindle are on a common plane, which is perpendicular to the B axis. Each of the two spindles has its own drive, which rotates them. Other distributions of the axes as well as an exchange of place between top and bottom are, of course, also possible.
So that the relative velocity between the shaping tool and the lens is as optimal as possible at every point on the surface of the lens, one of the two spindle axes must be set at an angle during polishing. In the polishing machines conventionally used today, this is accomplished by pivoting the above-cited, top-mounted pivoting head, to which the tool spindle is attached. The tool spindle is thus set at an angle. The diameter of the shaping tool is approximately twice that of the lens, which is laid against the tool in such a way that its edge does not project beyond the center of rotation of the shaping tool. For geometric reasons, the point of intersection of the two spindle axes must coincide with the center of curvature of the lens, which, after the pivoting head has been set at an angle, can be accomplished by moving the X slide and the Z slide.
The side of the shaping tool which rests against the lens during polishing is lined with a soft material. By adding a polishing suspension, which contains fine, solid particles, to this soft lining, material can removed from the lens in the desired way. Although essentially only the surface roughness is reduced during polishing, nevertheless a certain correction of the lens shape also occurs in the high-precision range. For this reason, the polishing tools must be made very precisely and must be reworked repeatedly as a result of the wear which occurs during polishing by means of a so-called "dressing" tool. In the past, the workpiece holder had to be removed from the workpiece spindle so that the dressing tool could be attached to this spindle. As a result of the natural tolerances of the participating components, tolerance errors developed with respect to the axial positions of the workpiece holder, dressing tool, and contour of the shaping tool when the components were changed. These tolerance errors in the axial position of the participating components led to loss of lens precision.
Dressing tools have recently become available which are combined with a workpiece holder; these are referred to below as "combination tools". A corresponding device is described in European patent application Serial No. 96-107,870.6, published as EP 0 807 491 A1, corresponding to U.S. Pat. No. 5,951,375. When these combination tools are used, the above-cited tolerance problems are avoided, because it is no longer necessary to replace the workpiece holder with a dressing tool to dress the shaping tool.
Nevertheless, the polishing processes normally used today still suffer from a significant disadvantage (even when the combination tools are used), namely, that the lens to be processed must be removed from the workpiece holder after the first surface has been polished and turned around so that it can be laid in a second polishing machine with the proper shaping tool for the second lens surface. This process is time-consuming and also carries with it the danger of damage to the first, already finished, surface of the lens.
So that the first, already finished lens surface is not damaged by the workpiece holder while the second surface is being polished, it must be provided with a coating of protective lacquer. It is time-consuming the apply this lacquer, and it is also time-consuming to remove it afterwards. In addition, undesirable drying times and environmental problems are also associated with the use of solvent-containing lacquers. The application of lacquer coatings with a highly uniform thickness, furthermore, is desirable with respect to the precision of the lens, but it difficult if not impossible to achieve this in practice.
In the processes normally used today to polish lenses, a workpiece holder is used which consists essentially of a hollow cylinder of plastic, the inside diameter of which matches the size of the lens, so that the lens can be inserted into it. In its interior, the hollow cylinder usually has a rubber membrane. Pressure can be applied to this membrane to press it flat against the lens. As a result, the lens is pressed against the shaping tool. Because of the special properties and inaccuracies of the membrane, this applied pressure between the lens and the shaping tool suffers from small irregularities across the surface. This is a disadvantage of the known lens polishing devices, which again has an unfavorable effect on the precision of the lens.
Devices have also become known in which this membrane is eliminated. Instead of lying on a membrane, the lens rests by its edges on a small shoulder in the tool holder at first. When compressed air acts on the back surface of the lens, it lifts the lens from the collar and presses it against the shaping tool. Although the pressure distribution is now more uniform that it was when a membrane was used, other disadvantages are encountered. Because air is compressible, the force holding the lens against the shaping tool is not as stable as would be desired. In addition, the air which flows through the gap between the lens and the workpiece holder exerts a drying effect on the polishing suspension around the edges of the lens, which is highly disadvantageous because of the cleaning work it entails. It is also unfavorable that compressed air is unable to dissipate adequately the heat of friction which develops during polishing, which means that the lens heats up unevenly. This can lead to distortion as a result of thermal expansion and thus to inaccuracies.